This invention relates to a method and apparatus for treating pressurized drilling fluid returns from a well, and in particular a method of extracting a portion of the gas component of the drilling fluid returns.
When drilling a well, whether it be through the use of overbalanced, balanced or underbalanced drilling techniques, pressurized drilling fluid returns must be extracted from the well and treated for purposes of separating, recycling, processing or otherwise disposing of the fluids and their constituent parts. Depending upon the nature of the drilling process being utilized and the geology through which the well is drilled, the drilling fluid returns may include a wide variety of solid, liquid and gas components, such as oil, water, rock particulates, natural gas and other hydrocarbon gases, etc. Regardless of the particular drilling operation, the drilling fluid returns extracted from a well can be at a relatively high pressure when compared to atmospheric conditions. The composition and pressure of the returns therefore necessitate that they be treated and separated before further use or disposal.
To date others have suggested a wide variety of different techniques and devices that may be used to treat pressurized drilling fluid returns. Such devices function to varying degrees of efficiency and, to a large extent, have their own specific advantages and disadvantages.
In spite of the various and different techniques that have been proposed for treating pressurized drilling fluid returns, prior devices and methods continue to suffer from a number of common and inherent limitations that severely effect their application, efficient operation, and cost. For example, in a closed high pressure separation system a significant problem that is commonly faced is one of erosion of parts of the system. This occurs when solid particles, in combination with gases and liquids, contact system components as they are propelled and travel through various piping and pressure vessels. The incidence and rate of erosion is accelerated by disturbances in flow patterns that may be caused by changes in flow area and direction, or through passage through valves, orifices or similar structures that cause pressure drops. Typically erosion will be most dramatic directly downstream of such a disturbance. Where high pressure drilling fluid returns are being processed and separated, erosion of piping or other structural elements can present a significant safety concern due to the potential for ruptures or failures of eroded parts. Erosion has the affect of significantly adding to the cost of a treatment system for pressurized drilling fluid returns due to the necessity for increased replacement of damaged components, and due to increased down-time when such components must be replaced.
A second limitation of presently known treatment systems concerns the inability to effectively and economically deal with high volumes of gas separated from drilling returns. In most instances separated gas returns cannot be easily stored, and are usually at lower pressures than production or sales pipelines. For this reason the separated gases are most often sent to flare. Flaring the gas is economically wasteful and environmentally undesirable. If low pressure gas is recovered from a separator, it must be sent to a gas compression system to increase its pressure to pipeline or process pressures, thereby adding to both capital and operating expense.
To combat situations where a large volume of gas is expelled from a well (for example when drilling through high deliverability gas zones) prior art systems and devices are typically constructed with a size and capacity that far exceeds the normal daily output from the well. This practice presents yet a third significant limitation under the prior art as over sizing separation devices in such a manner adds to the capital and operating costs of land based drilling systems. In addition, the sheer size and weight of such prior art systems in many cases also severely limits their application to offshore drilling projects.
Further, prior methods of treating drilling returns often involve monitoring liquid levels upstream of the choke that feeds a primary separation vessel as a means of regulating gas flow through the system. However, where the well expels a large quantity of liquid at one time (referred to as a liquid slug) such liquid monitoring methods are typically overloaded and fail, resulting in a liquid carryover into the gas stream. In addition, once again the size of such currently used systems results in increased costs and limited application in offshore drilling.
The invention therefore provides a method of treating pressurized drilling fluid returns from a well that helps to address some of the deficiencies in the prior art. The invention provides a method of treating such drilling fluid returns in a manner that reduces erosion of component parts of the system, enhances the ability to recover and utilize separated gas components at wellhead pressures from the returns, and that allows for the use of more compact equipment than many prior art devices, thereby helping to minimize capital costs and enhancing utilization in both land and offshore applications.
Accordingly, in one of its aspects the invention provides a method of treating pressurized drilling fluid returns from a well having a returns outlet, the method comprising the steps of providing a primary separation means for selectively separating solid, liquid and gas components of the drilling fluid returns, said primary separation means having input means for introducing drilling fluid returns thereto; providing a secondary separation means for selectively separating at least a portion of a gas component in the drilling fluid returns, said secondary separation means having input means for introducing drilling fluid returns thereto; connecting said input means of said primary separation means and said input means of said secondary separation means to the returns outlet of the well, said input means of said secondary separation means connected to the well returns outlet upstream of said input means of said primary separation means; providing control means to regulate the flow of drilling fluid returns through said input means of said primary separation means and through said input means of said secondary separation means; monitoring the flow of the gas component of the drilling fluid returns expelled from said primary separation means; and, in response to changes in the flow of gas expelled from said primary separation means, operating said control means to regulate the flow velocity of drilling fluid returns through said primary and said secondary means to thereby maintain the flow of gas expelled from said primary separation means within predetermined parameters.
In a further aspect the invention provides a method of treating drilling fluid returns from a pressurized source, said drilling fluid returns containing at least one of a gas component, a liquid component and a solid component, the method comprising the steps of; connecting a primary separator and a secondary separator to the source of pressurized drilling fluid returns, said secondary separator connected to the source of drilling fluid returns upstream of said primary separator; providing at least one flow controller to regulate the flow of drilling fluid returns through said primary and said secondary separators; monitoring the volume of the gas component of the drilling fluid returns separated by said primary separator; and, operating said flow controller to regulate the flow velocity of drilling fluid returns through said primary and said secondary separators in response to changes in the volume of gas separated from said drilling fluid returns by said primary separator to maintain the volume of gas separated from said drilling fluid returns by said primary separator within pre-determined parameters.
In yet a further aspect the invention provides an apparatus for treating pressurized drilling fluid returns from a pressurized source, said drilling fluid returns containing at least one of a gas component, a liquid component and a solid component, the apparatus comprising; a primary separator having an input port for introducing drilling fluid returns therein, and having at least one gas output port; a secondary separator having an input port for introducing drilling fluid returns therein, and having at least one gas output port; at least one flow controller to regulate the flow of drilling fluid returns through said primary and said secondary separators; at least one flow meter to monitor, and to generate a signal pursuant to, the volumetric flow of the gas component of the drilling fluid returns separated by said primary separator; and, a system controller, said system controller regulating the operation of said flow controller in response to signals generated by and received from said flow meter to thereby regulate the flow of drilling fluid returns through said primary and said secondary separators in response to changes in the volume of any gas separated from said drilling fluid returns by said primary separator and to thereby maintain the volume of any gas separated by said primary separator within pre-determined parameters.
Further aspects and advantages of the invention will become apparent from the following description taken together with the accompanying drawing.